Germany's interior minister, Thomas de Mazière, called the device a "real-test suitcase" that at no time put passengers' lives in danger.

He said it was not yet clear who was behind the test, and that a German investigation was in progress.

De Mazière could confirm only that the suitcase containing the suspicious package had been manufactured by a US company specialising in alarm systems.

"This company is a manufacturer of alarm and detection systems, and these real test suitcases are built to test security measures," he said.

According to tests by experts from the German federal criminal police, the suitcase did not contain explosives.

German security experts said today it was most likely that either US or African authorities were behind the test, following the recent security scare over parcel bombs sent from Yemen to the US.

The suitcase – containing batteries connected to a detonator and a ticking clock – was intercepted by authorities at Windhoek airport on Wednesday night and sparked an international terror alert. The Munich-bound Air Berlin plane, on which it was believed the suitcase was due to be loaded, was delayed for around eight hours while extensive security checks were carried out. Passengers were questioned by police when the plane landed at Munich airport in the early yesterday.

Initial theories that the bomb might have been a dummy run by terrorists were dismissed by US security sources interviewed by German television who said such "dry runs" were not typical.

But the new development had no impact on a nationwide security alert issued by de Maizière on Wednesday over "concrete information" that an Islamist terror cell was planning to attack Germany towards the end of this month.

Security measures have been tightened across the country. Extra armed police and military personnel have been sent to guard airports, railway stations, seaports and luxury hotels.

Several unattended suitcases and packages found at railway stations in Berlin today proved to be harmless, but the atmosphere in public places remained jittery.

Christmas markets, a magnet for tourists from around the world, are due to open on Monday. The police force has been told all holiday is to be cancelled until further notice because of additional security demands.